My sister has a 70's Bertram 26 with twin straight 6's. Just got the motors back after a 3 year motor rebuild debacle. Less than 1 hour on new motors developed a knock. Turns out one of the pistons/wrist pin went. side of piston is majorly scored. She spent another grand to have motor pulled, and piston replaced. 45 minutes on motor being very easy on it and it has the same knock, different hole. Tried adjusting valves, swapping lifters etc to elimante other possibilities and it seams to have eaten another piston. Whats going on??? I did notice the pins are slightly offset on the pistons..

no expert on them but have worked on a couple. I assume they are 292 ci? I know on the sixes a lot of guys use 305 V8 pistons in them. when you do you have to put two in backwards so the offset is the same. that is when using symetrical surface pistons like the ones with four valve reliefs. have you had the head off to look at the piston tops? there is usually a mark on them to tell you which way is front so if they used the 305 pistons two should be backwards. that is at least a start.

Check to see that the timing mark is correct at TDC. Balancers outer ring has been known to slip on the hub. I have seen them off as much as 18deg.

need to see pictures of the pistons and cylinder walls..with out looking at pics it might be a fuel issue..thease fuels now a days burn quicker and the carb may beed bigger jets..also piston to wall clearance may be minimal and when the engine heats up the cylinders swell..your line hone could be off as well,,that will distroy pistoms in a hart beat..

I cant get piks till the weekend. One side of the piston is heavily scored, the other side looked ok...

You back again jackwad....

Sounds like the wrist pins were set up way too tight, loading one side, possibly an oiling problem to the pins (does it have squirt holes in rods, aimed the wrong way) $.02

Does it use Offset Pins? Bad Rod Angle ?? IDK